Electrolyte and method for electrodepositing rhodium-ruthenium alloys

ABSTRACT

Disclosed is an electroplating bath suitable for depositing a rhodium-ruthenium alloy containing at least 90 weight % rhodium. Rhodium and ruthenium are present in the plating bath in the form of soluble platable compounds in a weight ratio of from 10:1 to 200:1. The deposits obtained by this bath exhibit improved brightness over comparable baths even without the addition of lead, and also exhibit reduced stress as compared to deposits obtained from rhodium-platinum baths.

United States Patent Stevens et al. July 1, 1975 [54] ELECTROLYTE AND METHOD FOR 3,515,651 6/1970 Ohkubo 204/43 N ELECTRODEPOSITING 3,692,641 9/1972 LOSi et al. 204/43 N RHODIUM-RUTHENIUM ALLOYS [75] Inventors: Peter Stevens, Parsippany; John M. Primary Examiner-G. L. Kaplan Deuber, Nutley, both of NJ. Attorney, Agent, or Firm-Arthur E. Kluegel; Richard [73] Assignee: Oxy Metal Industries Corporation, Mueller; Claeboe Detroit, Mich.

[22] Filed: July 29, 1974 [57] ABSTRACT [21] App]. N0.: 492,578

Disclosed is an electroplating bath suitable for depos- Related Apphcanpn Data iting a rhodium-ruthenium alloy containing at least 90 [63] Continuation-impar 0f e June weight rhodium. Rhodium and ruthenium are pres- 1973, abandoned. ent in the plating bath in the form of soluble platable 1 compounds in a weight ratio of from 10:1 to 200:1. [52] U.S. Cl 204/43 N The deposits obtained by this hath exhibit improved [5 l Illt. Cl C23b 5/32 brightness over comparable bathS even without the [58] Fleld of Search 204/43 N, 47, 123 dition of lead, and also exhibit reduced Stress as pared to deposits obtained from rhodium-platinum [56] References Cited baths UNITED STATES PATENTS 2,057,638 10/1936 Zimmermann et a1. 204/47 5 7 Claims, N0 Drawings ELECTROLYTE AND METHOD FOR ELECTRODEPOSITING RHODIUM-RUTHENIUM ALLOYS CROSS-REFERENCE TO RELATED CASES This application is a continuation-in-part of U.S. Ser. No. 372,375, filed June 21, 1973, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to improvements in and relating to the electrodeposition of rhodium alloys.

Electrodeposited rhodium has found considerable favor in recent years as a decorative coating material and as a contact face material owing to its extreme hardness and its resistance to corrosion and tarnish. Under normal conditions, rhodium is substantially unaffected by contact with common acids and chemicals.

Deposits of rhodium typically exhibit desirable characteristics with regard to resistance to corrosion and wear, and superior hardness coupled with exceptionally high resistance to heat and good electrical conductivity. These properties render to rhodium deposit useful not only as protective coatings for silverware and jewelry but also as protecting parts for electronic applications. The primary disadvantages of rhodium deposits are that they are somewhat lacking in desired brightness for certain applications, and that the deposits exhibit a high internal stress, particularly when deposited as a relatively thick layer, which results in surface cracks.

US. Pat. No. 3,671,408 discloses a bath suitable for depositing a rhodium-platinum alloy. The rhodiumruthenium deposit of the present invention is brighter than the rhodium-platinum deposit and the bath of the present invention will produce a bright deposit over an extended period of time without the necessity of adding small quantities of lead or similar brighteners as is necessary in the rhodium-platinum bath. In addition, the rhodium-ruthenium deposit of the present invention exhibits much less stress than the rhodium-platinum deposit.

U.S. Pat. No. 3,692,641 teaches a deposit containing ruthenium, rhodium and platinum. The deposit is predominately ruthenium and contains minor portions of rhodium or platinum. Additionally, the deposit disclosed in that patent contains indium, thallium or gallium. Since the deposit of that patent contains only minor portions of rhodium, it does not exhibit the aforementioned advantageous characteristics of a rhodium deposit.

US. Pat. No. 3,31 1,547 discloses a rhodium alloy deposit containing indium as the alloying metal. Unlike indium, ruthenium is a precious metal and the presence of ruthenium yields a brightened deposit.

SUMMARY OF THE INVENTION It has now been discovered that a rhodium deposit of improved brightness and stress resistance can be obtained by including in the electroplating bath a minor quantity of a soluble ruthenium compound. The bath is a substantially platinum-free aqueous acidic bath containing a soluble rhodium compound and a compatible soluble ruthenium compound in a weight ratio of rhodium to ruthenium which under normal plating conditions is sufficient to deposit an alloy of at least 90 weight rhodium.

DETAILED DESCRIPTION OF THE INVENTION The scope of the invention includes an electroplating bath, a method of electroplating comprising electrolyzing the bath, and the resulting plated article.

Since it is desired that the deposit exhibit, to the extent possible, the advantageous characteristics of a pure rhodium deposit, the deposit should be predominately rhodium. By predominately, we mean the deposit should contain at least weight rhodium. Preferably, the deposit will contain at least weight rhodium. The deposit should be substantially platinum-free, aside from the platinum impurities, because it has been found that the presence of platinum in the deposit contributes to increased stress and consequent cracking of the deposit. The ruthenium content of the electroplating bath is dependent upon the rhodium concentration. The rhodium concentration may be between 0.1 and 15 grams per liter and preferably between 0.5 and 5 grams per liter. The desired ruthenium content may vary somewhat depending on the presence and concentration of additive components and operating conditions such as current density but should be selected to provide a deposit of at least 90 weight rhodium. Under normal operating conditions specified below, the weight ratio of rhodium to ruthenium in the plating bath is between 10 and 200:1. Preferably, the ratio is at least 20:1 and most preferably at least 40:1.

The rhodium salts generally used for plating are rhodium sulfate, rhodium sulfamate or rhodium phosphate. However, any rhodium salt may be used which provides the rhodium in a condition such that rhodium can be plated from an acid bath and which is stable under the acid conditions required.

Suitable ruthenium compounds are those which are soluble in the aqueous acidic medium of the bath, which do not contribute any interferring metal ions, and which are compatible with the other bath components selected. By compatible, it is meant that the ruthenium compound selected does not interreact with other bath components so as to interfere with the plating process. The preferred ruthenium sources are ruthenium sulfate, hydrochloroperrutheneous acid, ruthenium trichloride and an alkali u-nitrido-bis [tetrachloroaquaruthenate (IV)]. The latter two compounds are the most preferred. An example of the nitrido compound is K N[RuCl H O] which may be obtained by reacting sulfamic acid and ruthenium trichloride.

The bath should be maintained on the acid side by the presence of at least 10 g/l of a compatible strong inorganic acid. Preferably, the bath contains sulfamic acid in a concentration of from 10 g/l up to the solubility limit of the solution. Preferably, the sulfamic acid is present in a concentration between 20 and 150 grams per liter and most preferably between 50 and 125 grams per liter.

The bath may additionally contain other additives well-known in the art. Suitable additions include those for improving conductivity, providing buffering power, and complexing metallic ions. It has been found that concentrated sulfuric acid may be advantageously added to the present bath to improve the conductivity. Normally quantities of up to milliliters per liter of solution and preferably between 1 and 50 milliliters per liter of solution are satisfactory.

The rhodium-ruthenium bath may be used to plate any base metal surface on which rhodium itself can be plated. Suitable base metals include brass, bronze, copper, silver, mild steel, nickel and nickel alloys. In some instances, it may be desirable to flash plate a layer of silver over the base metal prior to plating in order to avoid reaction between the base metal and the plating bath. Flash platings of palladium, palladium-gold or gold are also suitable for this purpose.

The normal operating conditions for deposition of the rhodium-ruthenium alloy are a temperature of between about 60 and 180F and a current density of about 5 to 40 amps per square foot. Preferably, the current density is less than 30 amps per square foot and the temperature between about 80 to 130F.

The deposits obtained by the process of this invention range from flash coatings of approximately 2 micro inch thickness up to coatings of 200 micro inches or more. The advantages of the deposit of this invention lie in the fact that both the brightness and low-stress are superior to deposits known in the art for rhodium and predominately rhodium alloy deposits. The thicker the deposit, the more important the stress-free characteristics of the deposit become.

EXAMPLE A plating bath was prepared by dissolving the following ingredients in one liter of water:

Sulfamic acid 100 grams Concentrated Sulfuric acid 25 grams Rhodium as the sulfate 2 g Ruthenium as Panels composed of brass were immersed in the bath as a cathode. The temperature was maintained at 150F and a current density varied between 10 and 20'amps per square foot was applied. The deposits were fully bright, white, even at thicknesses in excess of 200 micro inches. The plating rate was 3 to 4 milligrams per amp-minute. While foil deposits obtained from a comparable rhodium-platinum bath exhibited considerable stress cracking, the rhodium-ruthenium deposits exhibited no such cracking.

What is claimed is:

l. A substantially platinum-free aqueous acidic electroplating bath suitable for depositing a bright, lowstress rhodium alloy at a temperature of 60 180F and a current density of 5 to 40 amps per square foot, the bath comprising 0.1 to 15 g/l of rhodium as a soluble compound and a compatible soluble ruthenium compound in a weight ratio of Rh:Ru of from 10 to 200:1.

2. The bath of claim 1" additionally comprising at least 10 g/l of sulfamic acid.

3. The bath of claim 1 additionally comprising up to ml/l of concentrated sulfuric acid.

4. The bath of claim 1 wherein the weight ratio of Rh:Ru is at least 20:1.

5. The bath of claim 1 wherein said soluble rhodium compound comprises rhodium sulfate and said soluble ruthenium compound comprises an alkali ,u-nitrido-bis [tetrachloroaquaruthenate (IV) 6. A method of plating of predominately rhodium deposit on a base metal surface comprising electrolyzing the surface in the presence of the bath of claim 1.

7. The method of claim 6 wherein said base metal surface is selected from the group consisting of brass,

bronze, copper, silver, steel, nickel and nickel alloys. 

1. A SUBSTANTIALLY PLATINUM-FREE AQUEOUS ACIDIC ELECTROPLATING BATH SUITABLE FOR DEPOSITING A BRIGHT, LOW-STRESS RHODIUM ALLOY AT A TEMPERATURE OF 60* - 180*F AND A CURRENT DENSITY OF 4 TO 40 AMPS PER SQUARE FOOT, THE BATH COMPRISING 0.1 TO 15 G/L OF RHODIUM AS A SOLUBLE COMPOUND A COMPATIBLE SOLUBLE RUTHENIUM COMPOUND IN A WEIGHT RATIO OF RH:RU OF FROM 10 TO 200:1.
 2. The bath of claim 1 additionally comprising at least 10 g/l of sulfamic acid.
 3. The bath of claim 1 additionally comprising up to 100 ml/l of concentrated sulfuric acid.
 4. The bath of claim 1 wherein the weight ratio of Rh:Ru is at least 20:1.
 5. The bath of claim 1 wherein said soluble rhodium compound comprises rhodium sulfate and said soluble ruthenium compound comprises an alkali Mu -nitrido-bis (tetrachloroaquaruthenate (IV)).
 6. A method of plating of predominately rhodium deposit on a base metal surface comprising electrolyzing the surface in the presence of the bath of claim
 1. 7. The method of claim 6 wherein said base metal surface is selected from the group consisting of brass, bronze, copper, silver, steel, nickel and nickel alloys. 